1. Field of the Invention
This invention relates to a distributed-feedback semiconductor laser effecting single longitudinal mode oscillation.
2. Description of the Prior Art
A distributed-feedback semiconductor laser (hereinafter referred to as "DFB laser") has been examined which is provided with a grating inside the device in order to control the longitudinal modes of the semiconductor laser. In the conventional DFB laser, two modes having the same threshold gain exist in principle. In practical oscillation, oscillation is made in either one of these two modes. Therefore, it has been difficult to obtain single longitudinal mode oscillation with high reproducibility because of the structural variance when fabricating the laser devices.
Typical prior art references relating to the DFB laser are listed below:
(1) T. Matsuoka et al., Electron Lett., 18, 28(1982) PA1 (2) S. Akiba et al., Jpn. J. Appl. Phys., 21, 1736(1982) PA1 where n.sub.eff is an effective refractive index, which is given by the following formula: EQU n.sub.eff =.beta./R.sub.0 PA1 where R.sub.0 =2.pi./.lambda.,.lambda. is an oscillation wavelength and .beta. is a propagation constant. PA1 where L is a cavity length.